Tension control for a moving web



April 23, 1968 P. DE HERTEL EASTCOTT 3,379,390

TENSION CONTROL FOR A MOVING WEB Filed Nov. 16, 1966 2 Sheets-Sheet 1 AMPLIFIER f-IUID PRESSURE EGULATOR '6 FIG.]

TO FAN 84 -WIDTH OF FREE LOOP D=2R R T R 42 H L J Y 9 I x G VT FIG.3

FIG.2,

PETER def/ERTEL. EASTOTT INVENTOR.

BY TFQLEEP Rf nT Agent April 23, 1968 P. DE HERTEL EASTCOTT 3,379,390

TENSION CONTROL FOR A MOVING WEB Filed Nov. 16, 1966 2 Sheets-Sheet 2 74 PETER Je/IL'R'TCL EASTcarT INVENTOR.

RITnT Agent United States Patent TENSION CONTROL FOR A MOVING WEB Peter de Hertel' Eastcott, Peterborough, Ontario, Canada,

assignor to Canadian General Electric Company Limited, Toronto, Ontario, Canada, a corporation of Canada Filed Nov. 16, $66, Ser. No. 594,721 7 Claims. (Cl. 242-75.44)

ABSTRACT 0F THE DISCLOSURE The tension in a rapidly moving web of a pliable material such as paper is controlled by the following means: (a) a shallow loop is sucked in the web by means of air at a constant, relatively low negative pressure; (b) the curvature of the loop is measured by means of light directed onto the loop at an acute angle to its surface so that light varying in amount with loop curvature is reflected from the surface onto light sensitive cells; and (c) the variable electric signal obtained from the cells is used for controlling web tension.

This invention relates to the handling of a Web of pliable material such as paper. In particular, the invention is directed to controlling the tension in a web of paper moving at high speed.

It is well known that in processing materials such as paper, the tension in the moving web must be carefully controlled. As processing technology advances, the speed at which the web moves increases; consequently, tension becomes all the more exacting to control.

United States application for patent, Ser. No. 425,325, filed Ian. 13, 1965, Eastcott and Spibey, now abandoned, describes a high speed system in which a long or so called free loop is maintained in the web between its drive and brake. This loop is drawn into a parallel sided box by means of air in the box at a constant negative pressure. Since the loop is free to lengthen with slackness and shorten with tautness of the incoming web, it maintains constant tension in the web leaving the loop. Means associated with the box continually measures the length of the loop and gives an output signal representing the loop length; this signal is fed into a control system which controls the braking effort such that the length of the loop is maintained Within set limits.

It can be readily shown that the tension in the web from a free loop is :a function of the negative air pressure and the width of the loop; consequently, the loop must be relatively wide if the negative air pressure is to be kept low. A low ne ative air pressure is usually desirable because many qualities of paper such as, for example, newsprint contains weak spots which might otherwise rupture and leave blow holes in the sheet. A wide loop will, of course, require '21 relatively large suction box which takes up considerable space in the production line. This extra space is seldom available in existing web handling installations, and it is often impractical to modify the installation to take the box, even with the added advantages it offers. In a new installation, the production line can, of course, be laid out to take a box of the width necessary for a free loop.

The object of this invention is to provide in a web handling system apparatus for controlling web tension that employs a tension detector which is much smaller than the loop forming means described in the aboveidentified application and yet retains some of the advantages of the so-called free loop.

In the practice of this invention, the tension in a web of pliable material moving at high speed is controlled by means of the following: a shallow loop drawn in the 3,379,390 Patented Apr. 23, 1968 material, means for measuring the curvature or depth of the loop, and means for utilizing these measurements for tension control purposes. A loop restricted in depth to a relatively small arc of the free loop is drawn in the web transversely thereof by means of relatively low differential air pressure, and its curvature is measured by means of light directed onto the loop at an angle that allows some of the light to be reflected onto one or more cells having electrical characteristics altered by the intensity of the light, the quantity of light reflected varying with loop curvature. The signal obtained from the cells is modified to render it useful for tension control purposes. The shallow loop of this invention will be refer-red to hereinafter as a restricted loop to distinguish it from the free loop of the invention described in the above-identified Eastcott et a1. patent application. The curvature, i.e., radius of curvature, of the restricted loop is determined by the extent that the web deflects in forming the loop. A better understanding of the invention may be had from the following description of three specific embodiments taken in connection with the accompanying drawings in which:

FIGURE 1 is a diagram of a web handling system showing an enlargement of the tension detecting apparatus in cross section and the other components schematically;

FIGURE 2 is a view looking down into the cavity of the loop forming vapparatus;

FIGURE 3 is a diagram of a free loop illustrating the restricted loop as an arc of the semi-circular end of the free loop;

FIGURES 4 and 5 are other embodiments of the tension detecting apparatus; and

FIGURE 6 illustrates the use of rollers in the apparatus in place of stationary guide surfaces.

In the FIGURE 1 embodiment of the invention there is shown a system for regulating the tension in a web 10 of a pliable material such as paper released from a roll 11 and moving at high speed in the direction 12 over a tension detecting assembly 13. The web is unwound from roll 11, drawn over assembly 13 and wound onto a roll 11a by means of a motor M coupled to drive roll 11a. An electrical signal obtained in assembly 13 representing web tension is fed through a coaxial cable 14- to an amplifier 15 where it is amplified and fed through conductors 16 to a regulator 17 which controls the braking eifort applied to roll 11 according to the magnitude of the signal. The braking of roll 11 is by means of a fluid cylinder 20 energized from the regulator, slidable brake shoe 19 actuated by the piston in the cylinder, and a brake drum 18 attached for rotation with the roll and frictionally engaged by the shoe. Assembly 13 consists essentially of a pedestal 21 mounted on a foundation and supporting a fiat, horizontal base plate 22 on which there is mounted a draw table 23 directly :above the pedestal and a guard board 24 spaced to the left of the table. The table has a wall 25 rising vertically from the base plate and then curving to the right at 26; the guard board has a similar wall 27 spaced to the left of wall 25 and also rising vertically from the base plate but to a lower point 28 where it makes a right angle turn to the left and then curves downwardly at 29. An end wall located at each end of the draw table and guard board close the ends of the channel defined by walls 25, 27 and plate 22, one of the ends walls being shown at 30. The end walls may be simple plugs which rest in the channel at the edges of the loop and which are adjustable so the assembly can handle webs of different widths. They are low enough that the loop can pass freely over them without being obstructed by them, thereby allowing for variations in web width .and lateral movement. The end walls on channel define la box-like structure disposed transverse to web and having an open top side and a cavity 31.

The guard board and draw table are adapted to allow web 10 to be drawn freely over them as illustrated with a shallow loop such as that indicated at 32 or 33 drawn into cavity 31 by means of relatively low negative air pressure in the box. Vacuum may be applied to the box in any known way, as for example, by means of an exhaust fan at each end of the box having its intake in communication with cavity 31, one of which is illustrated at 84. The end walls 39 are located as closely as possible to the edges of the Web to minimize the amount of air drawn into the box at the ends of the loop. Curved surfaces 29 and 26 guide the Web into and out of the loop and maintain it in the proper position between the end walls of the box.

For any given pressure of the suction in the box, the curvature of the loop is an indication of the tension in the web; hence, by holding the pressure reasonably constant a signal representing the curvature of the loop can be used to control the braking etfort on roll 11. In the embodiment of the invention illustrated in FIGURES 1 and 2, a beam of light 34 from a lamp 35 is directed onto the undersurface of the loop at an acute angle thereto. Some of this light will be reflected down onto a light responsive device 36 located below the reflecting surface; the amount of light reflected will depend on the curvature or depth of the loop, the deeper the loop the less light falling on device 36. Lamp 35 is isolated from device 36 in an opaque box 39 having an intermediate opaque wall 41 dividing the box into a lamp compartment 37 and a device compartment 38. This box has a transparent top wall sloping in the general direction of the loop so as to allow light to emerge from compartment 37 and be reflected into compartment 38. Wall 41 is opaque and extends lengthwise of cavity 31 from the bottom and end walls of box 39 to window 40 so that no light can pass from compartment 37 into compartment 38 without going through the window. Compartment 38 may have an inner surface which reflects light entering the compartment onto device 36. Device 36 may be a group of solar cells responsive to the lighting level in the compartment and connected in series and to cable 14. Top 40 of box 39 may be a sheet of glass. As indicated in FIGURES 1 and 2, box 39 is a separate unit set-ting on base 22 in the middle of the suction box, its size depending on the size of the light source and the light responsive device. In general, it will pretty well fill the channel space under the loop as illustrated in FIGURE 1 land have a dimension ranging from 12 to 24 inches along the channel. It will be low enough that it will not interfere with the loop.

Since the electrical signal obtained at 36 is very small it is amplified at 15 in a conventional high gain amplifier. The amplified signal is then used for control purposes in regulator 17. Regulator 17 may be a hydraulic or pneumatic device wherein a relatively small electric signal can be used to control a relatively great fluid pressure in cylinder 20. Regulators suitable for this application are well known, one of which is the hydraulic regulator disclosed and claimed in United States Patent No. 3,266,376, Aug. 16, 1966, Eastcott et 21. In place of components 17 to 20, roll 11 may be braked by means of electrodynamic apparatus which has its output con-trolled by the signal from amplifier 15. This type of apparatus is also well known.

Regulation of the system is such that the tension in web 10 remains reasonably constant. If the tension increases above that desired, the loop will shorten from say position 32 to position 33; this increases the light falling on the solar cells, and as a result, the increased signal from the cells causes the regulator to reduce the fluid pressure in cylinder 20, thereby relaxing the braking effort on roll 11. A decrease in web tension causes the loop to lengthen and decrease the light falling on the cells, and this in turn leads to i creased braking cited.

It was pointed out earlier, that in the case of a free loop, web tension depends on the width or diameter of the loop and air pressure applied for maintaining the loop. If this loop is wide the air pressure will be low and vice versa. Because the loop is subjected to uniform air pressure, it will assume the shape of an arc of a circle, the shortest loop length possible. In other words, any restricted loop may be considered to be a portion of a free loop. Consequently, a relatively small arc of a wide free loop can be held by means of the same low air pressure as required for the free loop of the same curvature. Although the restricted loop lacks the web storage capabilities of the free loop and is unable to maintain web tension independently of loop length, it can be small and held with low air pressure, two important advantages in some web handling systems.

The web tension provided by a restricted loop can be determined from an analysis of FIGURE 3. In this figure numeral 42 designates the semi-circular end of a free loop of radius R and parallel sides 43, 44. The restricted loop now to be considered is represented by the are 45 terminated at H and G by surfaces such as those shown at 26 and 29 in FIGURE 1. In the free loop, web tension can be expressed as follows:

T=0.0181P.D

=0.0181P2R (l) where:

P is the vacuum in inches of water holding the loop;

D is the width of the loop in inches;

R is the radius of the loop in inches;

T is the tension in the web in pounds per inch of web transverse thereto; and

0.0181 is a constant.

i In the geometric figure illustrated in connection with are 45, HI and IG are opposite sides of a rectangle of length x, HI and JG are opposite ends of the rectangle and of length y, and z, the chord of the arc, is the length of diagonal HG of the rectangle. KL is a line from the center of semicircle 42 bisecting diagonal HG and perpendicular thereto. In the triangle KLG,

Substitution Equation 2 for R in Equation, 1, the following expression is obtained for tension:

The useful properties of the restricted loop as a tension measuring device may be readily ascertained from the foregoing expression. It is to be noted that the free loop will maintain web tension constant while providing web storage, but the restricted will provide very little storage; its main use is as a tension measuring device. The restricted loop is capable of very limited web storage only at the expense of tension changes. However, as a tension measuring device, the restricted loop offers the advantages of a small loop and low suction, enabling the loop forming apparatus to be relatively small. This is particularly important in web handling systems where the' machines must be located close together.

Another configuration of the loop forming assembly is shown in FIGURE 4. This assembly is essentially the same as that shown in FIGURE 1 except for the light source and the light responsive device. As before, a draw table 46, a guard board 47, a base plate 48, and a pair of end walls 49 define a cavity 50 into which a restricted loop 51 of web 10 is drawn when air is evacuated from the cavity as by way of an air outlet 52 at each end of the cavity. As the web advances in direction 12, it slides freely over the curved surfaces 53, 54, of the guard board and the draw table and sags into the cavity to form a shallow loop, transverse to the web. As in FIGURE 1, the end walls are located immediately below the loop near its ends so a limited amount of air will leak into the cavity through the spaces between these walls and the web.

In the FIGURE 4 assembly, the light source 55 is located in the draw table isolated from extraneous light so as to emit a beam of light 57 into the cavity through a transparent window 56 in the cavity defining wall of the table. Preferably, this light is in a collimated beam which strikes the undersurface of loop 51 at an acute angle thereto such that some of it is reflected down onto the light responsive device 58 located at the bottom of an enclosure 59 having a transparent upper wall 60 but otherwise opaque. As in FIGURE 1, the light source and the light responsive unit may be located at the center of the web, the light source being mounted on the table and the enclosure for the light responsive device resting on the base plate beside the guard board. Device 58 may again be a group of solar cells connected together for the most effective response to the level of illumination inside the opaque enclosure and the combination connected to cable 14.

The amount of light falling on device 58 depends on the length of loop 51. More light falls 0n the device when the loop is short than when it is long. Hence the electric signal generated by the solar cells will vary in magnitude according to loop curvature.

A third configuration of the loop forming assembly is shown in FIGURE 5. This assembly has a guard board 61 and a draw table 62 in the form of a flat plate extending from the base of the guard board to the right and ending in a curved trailing edge portion 63. The right hand wall of the guard board and the flat upper surface of the table define a right angular channel 6-4 transverse to web 10 into which a loop 65 of the web is drawn as it advances over the guard board and table in the direction indicated by arrow 12. Differential air pressure is used to hold loop 65 in the web; preferably, negative air pressure is applied by withdrawing air from channel 64 through an outlet 86 in the table near each end of the channel. Although not shown in the figure, the ends of the channel are obstructed against inward air flow by a pair of end walls resting in the channel and over which the edges of the web can freely pass. The end walls and evacuating means are the same functionally as those described in connection with FIGURES 1 and 4.

A lamp 66 located inside the guard board near the middie of the web projects a beam of light 68 through a transparent window 69 in the channel defining wall of the guard board, the beam being directed in the general direction of web travel. This light strikes the lower surface of the loop some distance to the right of window 69 and some of it is reflected downward by the surface into an enclosure '70 depending from the draw table near the middle of the web. Enclosure 70 is an opaque box having an upper transparent wall 71 flush with the guide surface of the draw table and encloses a device 72 which is responsive to the level of illumination in the box. In this configuration, the web actually covers part of window 71 as it passes over the draw table; the extent of the window coverage depends on the curvature of the loop, the greater the radius of curvature the less the coverage and vice versa. Since the level of illumination in enclosure 70 depends on loop curvature, the signal from device 72 will represent web tension because tension and curvature are interdependent. In this particular case, the loop acts as a light reflector and also as a light valve for enclosure 70.

The stationary guide surface on one or both the guide board and the draw table can be replaced by a roller as illustrated in FIGURE 6. In this figure, both surfaces have been replaced by rollers. Rollers 73 and 74 have their axes transverse to the web and are spaced apart so that a loop 75 can be drawn in the web by means of differential air pressure, preferably suction applied between the rollers below the web. When suction is used for holding the loop in the web, the loop receiving cavity consists of rollers 73 and 74, a bottom wall 76 and a pair of end walls in the form of plugs resting on the bottom wall at the edges of the web so that loop can pass freely over them. Air may be exhausted from the cavity through a pair of outlets 77.

Although it is preferred to suck the loop in the web by means of negative air pressure, it is also possible to flow the loop in the web by means of air at a pressure above atmospheric or combine blowing wit-h sucking. The expression differential air pressure is intended to encompass all three. Blowing the loop is also applicable to either stationary guide surfaces or rollers, and is illustrated in dashed lines with the rollers in FIGURE 6. Essentially, the apparatus consists of a hood 78 placed over the web and its loop receiving cavity so the positive air pressure inside the hood causes the web to sag between the rollers to form the loop. One or more inlets 79 ma} be provided on the hood for admitting the air.

What I claim as new and desire to secured by Letters Patent of the United States is:

1. Apparatus for handling a web of pliable material having light reflecting properties comprising means for advancing the web lengthwise; means for placing said web in tension; a source of air capable of maintaining a relatively constant differential air pressure; means for applying said differential air pressure to said web on a transverse portion thereof for drawing a shallow loop in the web appearing in profile as an arc of a circle varying in curvature with the tension; a light source for projecting a beam of light onto the outer curved surface of said loop at an acute angle thereto; means for collecting light refiected from said surface, the amount of light reflected varying with said curvature; and means for utilizing at least some of the light collected for controlling said tension.

2. Apparatus for handling a web of pliable material having light reflecting properties comprising a draw table and a guard board mounted on base structure in spaced relation transverse to the web; a pair of end walls cooperating with said draw table, guard board, and base structure to define a cavity; means on the draw table side of the structure for drawing said web over said guard board, cavity, and draw table; means on the guard board side of the structure for applying a braking force to said web for placing the web between said braking and drawing means in tension; means for exhausting air from said cavity at a rate maintaining a relatively constant negative air pressure therein and thereby causing the web to sag into the cavity as a shallow loop appearing in profile as an arc of a circle extending from the guard board to the draw table, the tension in the web determining the depth of the loop or curvature of the are; a light source for projecting a beam of light onto the outer curved surface of said loop at an acute angle thereto; a light responsive device for receiving light reflected from said surface and providing an electric signal representing the amount of light reflected, the amount of light reflected varying with the curvature of the arc; and control apparatus utilizing said signal to control said braking force so as to maintain relatively constant tension in the web.

3. The apparatus defined in claim 2 wherein said light responsive device is a group of solar cells generating an electric signal from the light falling on them, and said control apparatus comprises amplifying means for amplifying the signal from said cells, a fluid regulator controlled by said amplified signal, a fluid actuator energized by the output from said regulator, and braking means operated by said actuator.

4. The apparatus defined in claim 2 wherein said light responsive device is a group of solar cells generating an electric signal from the light falling on them.

5. Apparatus for handling a web of pliable material having light reflecting properties comprising a pair of guide surfaces located transverse to the web in spaced relation; walls cooperating with said surfaces to define a cavity; means on one side of said cavity defining structure for drawing web over said surfaces and cavity; means on the other side of said cavity defining structure for applying a braking force to said Web for placing the web between said braking and drawing means in tension; means for exhausting air from said cavity at a rate maintaining a relatively constant negative air pressure therein and thereby causing the web to sag into the cavity as a shallow loop appearing in profile as an arc of a circle extending between the guide surfaces, the tension in the web determining the depth of the loop or curvature of the are; a light source for projecting a beam of light onto the outer curved surface of said loop at an acute angle thereto; a light responsive device for receiving light refiected from said surface and providing an electric signal representing the amount of light reflected, the amount of light reflected varying with the curvature of the arc; and control apparatus utilizing said signal to control said braking force so as to maintain relatively constant tension in the web.

6. The apparatus defined in claim 5 wherein at least one of said guide surfaces is a roller.

7. Apparatus for handling a web of pliable material having light reflecting properties comprising a draw table having a flat table surface with a transparent window therein; a guard board located in spaced relation to said window and having a guide surface displaced from said table surface, said guard board and draw table defining a channel transverse to the web; means for drawing web over said guide surface, channel and table surface; means for braking the web advancing over said surfaces for placing the web on the surfaces in tension; means for ap plying relatively constant differential air pressure to the web overlaying said channel for the formation of a loop in the web sagging into the channel, said loop terminating on said guide surface and on said table surface on said window such that a portion of the window is covered by the web and the remainder of the window is exposed under the loop, the extent of the window coverage varying with loop curvature or web tension; a light source for illuminating the region between said exposed window and said loop; a light receiving enclosure on the other side of said window for receiving light transmitted through the window from said region, the level of illumination in said enclosure varying according to exposed window surface; a light responsive device located in said enclosure and energized by light therein to produce an electrical signal representing the level of illumination in the enclosure; and control apparatus utilizing said signal to control said braking so as to maintain relatively constant tension in the web.

References Cited UNITED STATES PATENTS 2,900,143 8/1959 Bandy et al. 24275.44 3,047,198 7/1962 Long 24275.52 X 3,203,635 8/1965 Rayfield et a1. 24255.l2

FRANK J. COHEN, Primary Examiner.

M. MINTZ, Assistant Examiner. 

